Single chamber ionization smoke detector

ABSTRACT

An ionization smoke detecotor having a single open ionization chamber which is connected with a field effect transistor and wherein the leakage impedance between the gate and drain of the transistor serves as a high impedance path.

United States Patent Takahashi [451 July 1 l, 1972 [541 SINGLE CHAMBER IONIZATION SMOKE DETECTOR 5 [56] References Cited [72] Inventor: Naoki Takahashi, Yokohama, Japan UNITED STATES PATENTS 1 Assisnw Nman Company Limited, Tokyo, Japan 3,295,121 12/1966 Meyer ..340/237 s [22] Filed: Feb. 10, 1970 Primary Examiner-Archie R. Borchelt [2 1] Appl' Muss Attorney-Eugene E. Geoffrey, Jr.

[30] Foreign Application Priority Data [57] ABSTRACT April 25, 1969 Japan ..44/3 1475 An ionization smoke detecotor having a single open ionization chamber which is connected with a field effect transistor and [52] 11.8. CI ..250/83.6 FT, 250/44, 340/237 S h i h l k impedance b t the at a d drain of [5 l Int. Cl. ..G0ln 21/26 h transistor serves as a high impedance path. detector [58] Field of Search ..250/44, 83.6 FT, 43.5 D;

340/237 S 3 Claims, 4 Drawing Figures Oi 1 ---O\24' SINGLE CHAMBER IONIZATION SMOKE DETECTOR This invention relates to an ionization smoke detector which utilizes a chamber including a radioactive source and wherein the saturation current flowing through the chamber is reduced by the presence of smoke.

Prior known smoke detectors have utilized two ionization chambers each including a radioactive source. One of the chambers are sealed while the other chamber was open for the admission of smoke. The presence of smoke was indicated by the different effective impedances of the chambers when smoke was present in one of the chambers. With prior known detectors, however, the closed ionization chamber served effectively as a high resistance with the result that the structure was not only complicated but also materially increased the cost.

One object of the invention resides in the provision of a novel and improved ionization chamber which overcomes the difficulties heretofore encountered with prior devices and utilizes a single open chamber for the detection of smoke. According to the invention a field effect transistor is utilized with the source-drain path being connected across the terminals of a power supply. A single open ionization chamber including a radioactive source is connected between the gate electrode and one terminal of the power supply and the leakage resistance between the gate and drain electrodes is used as a high resistance path.

The above and other objects of the invention will become more apparent from the following description and accompanying drawings forming part of this application.

In the drawings:

FIG. 1 is a circuit diagram of a conventional ionization detector using a closed chamber and an open chamber;

FIG. 2 is a circuit diagram of a smoke detector in accordance with one embodiment of the invention;

FIG. 3 is a graph showing the operation of the detector of FIG. 2; and

HO. 4 is a circuit diagram of another embodiment of the smoke detector in accordance with the invention.

In the description like reference numerals have been utilized to identify corresponding components of the several figures.

Referring first to F IG. 1, a typical known ionization detector utilizes a closed ionization chamber 4 having a radioactive source 2 and an open ionization chamber 8 having a radioactive source 6. The two ionization chambers are connected in series between a conductor 10 attached to the positive terminal of the power supply and a conductor 12 attached to the negative terminal of the power supply. A field effect transistor 14 which has a high input impedance was connected across the power supply with the drain being connected to the conductor l and the source being connected through a load resistor 20 to the conductor 12. The gate 22 was connected to the junction 24 of the ionization chambers 4 and 8.

In the operation of the device when smoke entered the ionization chamber 8, its impedance increased and the voltage at the junction 24 changed with respect to the voltage on line 12. This change was applied to the gate electrode 22 and amplified by the transistor. The amplified output signal appeared at the output terminal 24' connected to the source electrode 18. To provide a closed ionization chamber such as the chamber 4 not only required an additional radioactive source 2 but also caused the structure to be more complicated and expensive. This invention as illustrated in FIG. 2 utilizes a single open ionization chamber 8 and utilizes the leakage impedance represented by the resistor 26 shown in broken lines between the gate and drain electrodes in place of the ionization chamber 4.

With the structure as shown in FIG. 2, the current flowing through the leakage impedance 26 decreases with an increase in the voltage V at the gate electrode 22 as shown by the broken line A in FIG. 3. In actual tests using a conventional junction type field effect transistor as the transistor 14, the leakage current only varied gradually with a change in the voltage V as shown by curve B in FIG. 3.

The saturation current, however, flowing through the ionization chamber 8 varied linearly and approximately proportionally to the gate voltage V within the range of a low applied voltage. The saturation current is relatively large and is illustrated by the line C of FIG. 3 in the absence of smoke. In the presence of smoke the current through chamber 8 is represented by the line D of FIG. 3.

Under normal conditions when smoke does not exist, the current flowing through the leakage resistance 26 also flows through the ionization chamber 8, and the gate voltage V is V When smoke enters the ionization chamber 8, the saturation current flowing through the chamber decreases so that the gate voltage of transistor 14 increases to V The voltage change V -V at the gate electrode caused by presence of smoke is amplified by the transistor, and an output signal appears at the terminal 24. This output signal may be utilized to drive a silicon-controlled rectifier or other device for the operation of a suitable alarm device or other indicator. With the ionization detector as described above it is evident that only one open ionization chamber is required so that only one radioactive source is needed, and the structure is substantially simplified.

A modified embodiment is illustrated in FIG. 4 and employs means for control of the sensitivity and also facilitates testing operations. Specifically the electrode 6 of the ionization chamber 8 is connected through a conductor 12' to the variable voltage source such as the potentiometer 28 which is connected between the conductors l0 and 12.

Since the voltage at the electrodes 6 can be changed by adjusting the potentiometer 28, the voltage at the gate electrode 22 of the field effect transistor 14 can be changed regardless of the presence or absence of smoke. When the voltage at the electrode 6 of the ionization chamber 8 is changed, the same effect is obtained as changing the bias voltage on the gate electrode 22 with the result that the sensitivity of the device can be controlled.

If the voltage at the electrode 6 is adjusted to an extreme value by the potentiometer 28, the voltage at the gate electrode 22 will have a value corresponding to the value experienced during the presence of smoke. Accordingly it is possible to test the operation of the detector without the actual introduction of smoke.

While only certain embodiments of the invention have been illustrated and described, it is apparent that alterations, modifications and changes may be made without departing from the true scope and spirit thereof as defined by the appended claims.

What is claimed is:

1. An ionization smoke detector comprising a field effect transistor having the source-drain path connected between reference potential points of a power supply and a single opentype ionization chamber including a radioactive source therein connected between the gate electrode of said field effect transistor and one of said reference potential points, said ionization chamber and the gate-drain leakage path forming a voltage divider, whereby the leakage impedance existing between the gate and drain of said field effect transistor forms a substantially constant current high resistance path within a selected range of gate voltages.

2. An ionization smoke detector according to claim 1, characterized in that one terminal of the open-type ionization chamber is connected to a variable voltage source to control the sensitivity and facilitate testing operations.

3. An ionization smoke detector according to claim 1 wherein said field effect transistor is of the junction type.

l IF l 

1. An ionization smoke detector comprising a field effect transistor having the source-drain path connected between reference potential points of a power supply and a single opentype ionization chamber including a radioactive source therein connected between the gate electrode of said field effect transistor and one of said reference potential points, said ionization chamber and the gate-drain leakage path forming a voltage divider, whereby the leakage impedance existing between the gate and drain of said field effect traNsistor forms a substantially constant current high resistance path within a selected range of gate voltages.
 2. An ionization smoke detector according to claim 1, characterized in that one terminal of the open-type ionization chamber is connected to a variable voltage source to control the sensitivity and facilitate testing operations.
 3. An ionization smoke detector according to claim 1 wherein said field effect transistor is of the junction type. 